For marine vehicle propulsion, propellers or impeller-driven water jets are mostly being used. The principle to generate thrust in water is to use some mechanism to build up water kinetic energy from the water velocity in line with the thrust axis. Marine propellers or marine impeller-driven water jets all depend on the spin of blades in water to build up the water kinetic energy. Because of the spin of blades in water, the kinetic energy built up in water is contributed not only from the water velocity in the thrust-producing axis but also from the water rotational velocity about the spinning axis. The kinetic energy from the rotational velocity of water doesn't contribute to the generation of thrust and therefore it is an energy waste. This principally-embedded energy waste leads to the fact that such propulsors could hardly reach close to the ideal efficiency of propulsor. The highly rotational water kinetic energy not only brings down the efficiency of the propulsor but also the sources of blade surface cavitations and the helical vortices in the propulsor flow wake that generate water noises. Further more, the increase of water velocity in the thrust-producing axis through the spinning of the blades works on the principle of a lifting foil. A foil requires an optimal angle of attack for maximum lift, likewise an optimal pitch angle of the blade is required to have a maximum increase of the thrust-producing water kinetic energy. For a given design of propeller or impeller-driven water jet, it could hardly operate in optimal pitch angle at all vehicle speeds, and that is why a propeller or an impeller-driven water jet can only reach its highest efficiency at the design point. As the vehicle operates at off-deign points, the efficiency of such propulsors degrades greatly. In other words, such propulsors could hardly offer the thrust power that is proportional to the input power. In real life, that fact reflects a poor acceleration of a marine vehicle equipped with such propulsors.
Our forefathers had long before understood that to most effectively propel and offer almost linear propulsion power to his boat one should do what oarsman does commonly seen in boat racings. In one propulsion cycle, oarsman gives a powerful stroke of his oar to expel or discharge the water, which generates a reaction force, i.e., the thrust on the oar surface to push the boat, and then follows an effortless oar recovering stroke through the air. The reason of such a propulsion cycle being highly efficient is that the mechanical work done on the oar to expel the water more or less only accelerates the water velocity in line with the thrust and the oar recovering through the air introduces negligible resistive energy loss. There also exists piston or reciprocating water jet propulsors to propel marine vehicles. From the way of expelling or discharging water, such propulsors work in the same principle of oars. Similarly, the biggest advantage of a piston water jet propulsor is that the mechanical motion of the piston is in line with the thrust-producing axis and therefore such a motion builds up the water kinetic energy only from the water velocity in the thrust-producing axis. Because of this reason, such a propulsor has a nearly constant efficiency at any working condition or vehicle speed. This characteristic of a piston water jet is consistent with the common knowledge that the efficiency of a positive displacement pump is nearly constant and higher than an impeller-driven pump of the same power.
Unlike the oar recovering through air, an issue relating to the efficiency of a piston water jet propulsor is the energy cost in the water intake process during the piston's recovering or back stroke. Prior arts of piston water jet propulsor all employ the water intake from the axial direction of the cylinder as the piston takes a back stroke, for example, through the openings on the piston. With this axial intake, the piston during its recovering stroke moves in a direct headwind of the intake flow, resulting in a large resistance to the piston's motion. The piston's mechanical work to overcome this resistance during the recovering stroke is an energy waste, which negatively affects the efficiency of the propulsor.